Image::Position: Fix pre/post increment/decrement
Image::Position: Fix pre/post increment/decrement

These were the wrong way around; the dummy int parameter codes for the post-increment / post-decrement operator.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3